Telecommunication networks can be divided into circuit switched and packet switched networks. In circuit switched networks, the transmission connection between the transmitting party and the receiving party is reserved before starting the transmission. It is a drawback of this connection method that the transmission connection is reserved even if no information is transmitted over the connection. In packet switched networks, transmission of packets may be connection-oriented or connectionless. In connectionless packet switched networks, the transmission network is common to all users. Information is transmitted in packets which contain information on their destination. Resources of the transmission network are not reserved in advance, nor are any packets transmitted when there is no information to be transmitted. In this way no transmission network capacity is reserved unnecessarily. In the connection-oriented packet switched technology, virtual circuits are formed for certain transmission routes between network elements and every packet of the connection is routed along the same route. Thus, the information is routed as in circuit switched networks, but no transmission capacity is unnecessarily reserved. E.g. the ATM (Asynchronous Transfer Mode) network is formed with the aid of virtual circuits.
Accessing packet switched networks like the Internet is further possible in many ways. The pan-European GSM mobile communications system, which is based on time division multiple access TDMA, allows circuit switched connection to packet switched networks in a known manner, when an adaptation protocol (e.g. a point-to-point protocol, PPP) is used between the mobile station and the connection point located in the network. The GPRS service (General Packet Radio Service) defined in the GSM system also allows packet switched connection from mobile stations to packet data networks like the Internet. FIG. 1 shows the simplified structure of the GPRS network. The serving GPRS support node SGSN contains mobility management and safety functions of the mobile stations. Through the gateway GPRS support node GGSN the network transmits packet data to and receives packet data from a packet data network PDN. By way of the gateway node the GPRS network is also in connection with other networks, such as the Internet or an X.25 network. Both the gateway and the serving node contain IP routing functions. Base station subsystem BSS includes base station controller BSC and one or more base stations BS. Mobile station MS is in connection with a base station over the radio path. In addition, the network includes home location register HLR for permanent storing of subscriber information.
In the GPRS network, transmission of packets between the mobile station and the serving node is transparent from the viewpoint of the base station system, and no record relating to the mobile station's GPRS service is stored in the base station system. Instead, every serving node SGSN has context information about the mobile stations which it is serving. In the GPRS system, context information can be divided into mobility management MM and packet data protocol PDP information respectively. The mobility management will tell where the mobile station is located and in which state it is. Possible states of a mobile station registered with a GPRS system are the idle state, the standby state and the ready state. When in the idle state, the mobile station is passive and it is able to receive only the broadcast of the base stations, but no point-to-point packets can be transmitted between the mobile station and the network. In the ready state, the mobile station is able to receive packets without any call procedure. From the ready state the mobile station moves to the standby state after a certain non-activity period. From the standby state the mobile station returns to the ready state after a call from the network or for transmission of a packet. The move between idle state and active state takes place by GPRS Attach and GPRS Detach procedures. In the ready state, the network knows the location of the mobile station with one-cell precision. Each packet to the mobile station is routed individually with the aid of a cell identifier attached to the packet from the serving node to the correct base station. Thus, user information is transmitted between the network and the mobile station without any connection assigned between the base station system and the serving node SGSN.
FIG. 2 shows a new third generation (3G) mobile communications system which is based e.g. on WCDMA (Wideband Code Division Multiple Access) technology. The 3G mobile communications system combines packet data transmission functionality with circuit switched communication. The serving node 3G SGSN corresponds to the serving node SGSN of the GPRS network, like the gateway node GGSN, which attends to connections with other packet data networks. Hereinafter in this application a serving node SGSN means the serving node of the 3G network. The mobile services switching centres 3G MSC attends to the routing of circuit switched traffic. The network may also be implemented without any mobile services switching centre 3G MSC. The subscriber information of mobile stations is stored permanently in home location register HLR and temporarily in visitor location register VLR located e.g. in connection with the mobile services switching centre, and in serving node SGSN. The radio network subsystem RNS includes a radio network controller RNC and at least one base station BS. The operation of the whole system is monitored by operation & maintenance system O&M.
In the third generation mobile communications system, the radio network subsystem must have information about mobile stations to be served over the radio interface, because the radio network subsystem allocates radio resources for the subscribers and it is able to combine both circuit switched and packet switched connections of the mobile station in the radio interface, using the same spreading code when e.g. WCDMA technology is used. It has been proposed that connections over the Iu interface between the radio network and the main trunk should be connection-oriented due to the said combining and coding of circuit switched and packet switched traffic. In addition, a connection set-up to be made for the packet service allows advantageous transmission to the radio network subsystem of parameters describing the quality of the service (e.g. transmission rate, transmission delay). Without a connection set-up which takes place separately, these parameters must be added separately to each packet to be transmitted. A connection between the serving node and the radio network subsystem can be implemented e.g. with ATM in such a way that for each mobile station a virtual channel is reserved according to a certain ATM adaptation layer (e.g. ATM Adaptation Layer 5 AAL5 or ATM Adaptation Layer 2 AAL2).
In order to combine circuit switched and packet switched traffic in a radio network subsystem, it is preferable to unify the mobile station state management. For this reason, an idle state and an active state are proposed as mobility management (MM) states of the mobile station registered with the 3G network. These states differing from the GPRS system are also natural states, when the transmission of packet data between the serving node and the radio network is connection-oriented, as presented above. In the idle state, the mobile station is only able to listen to the broadcast of base stations and to move into an active state through a procedure to be described later in connection with FIG. 3. In the active state, the mobile station can transmit and receive data packets continuously. When the mobile station is in the active state, a communication connection has been set up between the serving node and the radio network and at least management resources are reserved for the mobile station in the radio network. Thus, each mobile station reserves network resources for use by itself in the active state. Of the presented 3G mobile communications system it is known to release limited radio resources during pauses in the packet data transmission, whereby the mobile station in the active state moves from the radio channel dedicated to the mobile station to a common radio channel, and the allocated radio resource can thus be released for other use. The connection between mobile station and base station is continued on the common channel, on which it is possible to transmit small data packets in both directions and to receive broadcast of the base station in the mobile station. When the packet data transmission is resumed, the radio network subsystem dedicates a new radio channel for the connection between base station and mobile station. The main trunk is not aware of this change of the used type of radio channel, so the mobile station's state remains active from the viewpoint of mobility management (MM).
FIG. 3 shows signalling between a mobile station and a 3G mobile communication network in the beginning of a packet data transmission. To keep it simple, the figure does not show any state-of-the-art signalling between the mobile station and the radio network subsystem for setting up e.g. connections of lower layers. In the Attach procedure 31, the serving node SGSN e.g. fetches subscriber information of the mobile station from the home location register. After this procedure, the mobile station is able to transmit and receive short messages (SMS) and to listen to the broadcast of base stations, but the mobile station is not able to transmit or receive packet data, because no packet data address is yet available to it. Hereby the mobile station is in the idle state. In PDP Context activation procedure 32, a packet data address is indicated to the mobile station, whereupon the mobile station will also be able to transmit and receive packet data. The mobile station has hereby moved into the active state. By the PDP Context activation procedure of the packet data protocol the mobile station is thus committed to a packet data address or to several addresses. Either the mobile station or the network can start this procedure. Before packet data is transmitted between the network and the mobile station, a connection-oriented communication connection is set up between serving node SGSN and radio network subsystem RNS by Bearer request/assignment procedure 33, wherein identifiers relating to the connection are transmitted, such as connection rate requirements and transmission delay requirements for the radio connection. After the communication connection has been set up between radio network subsystem RNS and serving node SGSN, it is possible to transmit packet data between the serving node SGSN and mobile station MS through radio network subsystem RNS (point 34).
In the operation of the third generation mobile communications system it is a problem as regards the transmission of packet data that resources of the transmission path between serving node SGSN and the radio network subsystem as well as memory resources of the radio network subsystem are reserved unnecessarily due to the burst-like character of packet data. Since a connection can be set up e.g. in the morning and end only in the evening, an address is reserved on the transmission path and connection management resources are reserved in the radio network subsystem for the whole day, even if no packets to be transmitted were to travel in the virtual circuit. Thus, a connection-oriented connection as described above for the serving node and the radio network subsystem consumes the transmission network's limited address space and the radio network subsystem's management resources.